I (GP) can think of no more obvious way to illustrate the increasing intensity of cardio workouts than to number them - numbers going up as intensity does.

Having progressed from treadmill walking and a low to moderate intensity steady state treadmill workout, or already being of a reasonable fitness level, you should find the following workout fine. As normal, please monitor your heart rate to ensure you're training within your required heart rate zones.

So, here goes....Workout 1.

Treadmill Workout 1

Warm Up: 5 Mins @ 3-6 kph (no incline)

Main Workload: 3 sets of 2km with 3 minutes recovery (low intensity) in between.
Ease in to the first 2km set & then choose a challenging intensity for the second two.

Cool Down: 5 Mins Easy @ 6-3 kph (no incline)